Don't reset density when going to splitscreen if there was no override.

We only override density for freeform tasks if desktopConfig.useDesktopOverrideDensity is true. So only reset it if so.

Bug: 440245803
Test: Manual. Check splitscreen on external display and see apps are no longer at wrong density.
Flag: EXEMPT bugfix
Change-Id: Iddaaca39ce9c185c38560bfbf77b6d98f20f3510